It is well known for various end use applications to provide articles that are coated with a material that is characterized by wear or corrosion resistance superior to that of the body or substrate of the article. For this purpose, it is known to provide an alloy article, such as an iron, nickel, cobalt and titanium-base alloy, which is formed by various conventional operations, such as rolling, forging and extrusion, to a final-product configuration. Thereafter, the desired wear or corrosion-resistant coating is deposited. The coating is selected depending upon the wear or corrosive media to which the article is to be subjected during use. Typically, for this purpose, the coating is of a material that is harder and less formable than that of the article, and consequently if the entire article were made of the coating material or if coated prior to forming it would be difficult or impossible to form the article to the desired configuration. In addition, the resistant coatings are generally of a material more expensive than that of the remainder of the article. Typical coatings which are applied to these alloy substrates for wear and/or corrosion resistance are refractories, ceramics and intermetallic compounds.
With iron, nickel and cobalt base alloys, the deposited, resistant coatings are susceptable to separation from the substrate by spalling as a consequence of differential thermal expansion between the coating and the substrate. Wear and corrosion resistant coating materials typically have a coefficient of thermal expansion considerably lower than that of the alloy substrate. Thermal spalling therefore may occur during temperature changes, because the differential thermal expansion between the substrate and the coating creates stresses at the coating-substrate interface which may exceed the interfacial bond strength. In addition, spalling may occur due to mechanical stress superimposed on the coating during commercial use, e.g. impact loads. This propensity for spalling of these coatings is exacerbated by the inability of these coatings to relieve these stresses by plastic flow because of their typical low-ductility and high-hardness.
The coatings, which generally must be applied in accordance with conventional practices at elevated temperature, may also spall on cooling to ambient temperature after elevated-temperature application. Consequently, because of the spall problem many of the desirable wear and/or corrosion-resistant coatings are limited in their use to specific alloy substrates of limited commercial utility and when used may be restricted to undesirably thin coating thicknesses insufficent for prolonged use of the article in commercial applications.
With titanium-base alloys and articles made therefrom, the desired, well known strength-to-weight ratio of titanium is advantageous in various commercial applications. Titanium alloys, however, perform relatively poorly in applications requiring resistance to wear, erosion and abrasion. Consequently, wear, abrasion and erosion-resistant coatings for use with titanium-base alloys are commercially significant.
A desirable coating for this purpose is titanium diboride (TiB.sub.2). This compound is extremely hard and exhibits outstanding wear properties. Very thin layers of intermetallic compounds of titanium and boron, including titanium diboride, can be formed on titanium alloy surfaces by subjecting the titanium alloy to activated boron-diffusion processing at elevated temperatures. Unfortunately, the temperatures and times required to form these boride diffusion layers to depths or thicknesses of commercial significance are so high that degradation of the properties of the titanium alloy substrate results. Titanium diboride deposited or added-on coatings, however, as opposed to diffusion layers, may be produced on titanium alloy substrates by the use of chemical vapor deposition (CVD) in commercially sufficient thicknesses and at temperatures below which the titanium alloy substrate is degradated. Specifically, in accordance with conventional practice these coatings may be provided by hydrogen reduction of titanium tetrachloride and boron trichloride to form titanium diboride. Hydrogen chloride gas, however, is formed as a by-product of this reaction. Unfortunately, halogens and halogen-containing compounds, including chlorine and hydrogen chloride gas, corrode and otherwise degradate the titanium alloy surface so that the desired high-quality CVD coatings cannot be produced. Therefore, titanium base alloy articles having a titanimum diboride abrasion or wear resistant coating of adequate thickness for the desired commercial applications are not available.